M221 with no parameters report flow

Marlin/src/gcode/config/M221.cpp

     planner.flow_percentage[target_extruder] = parser.value_int();
     planner.refresh_e_factor(target_extruder);
   }
+  else {
+    SERIAL_ECHO_START();
+    SERIAL_CHAR('E');
+    SERIAL_CHAR('0' + target_extruder);
+    SERIAL_ECHOPAIR(" Flow: ", planner.flow_percentage[target_extruder]);
+    SERIAL_CHAR('%');
+    SERIAL_EOL();
+  }
 }